Electrodeposition of lead-tin-antimony alloys



Jan. 11,;1949.- J M, OE 2,458,827

ELECTRODEPOSITION 0F LEAD-TINr-ANTIMONY ALLOYS Filed May 10, 1946 INVENTOR ,AM M

ATTORNEY Patented Jan. 11, 1949 UNITED srAr ES PATENT OFFICE ELECTROD EPOSITION OF LEAD-TIN- 1 AN TIMONY ALLOYS James M. Booe, Indianapolis,Ind., assignor to P. R. Mallory &"Co., Inc., Indianapolis, Ind., a corporation of Delaware Application May 10, 1946, Serial No. 668,653

carrying the principles of the invention into practice Lead -tin-antimony alloys are used for bearings, printing and lithograph plates and other applications where a relatively hard and corrosion and wear resistant surface is required. Since the characteristic properties of this alloy are generally required for surface functions, the application of the lead-tin-antimony alloy in relatively thin layers would serve to lend its characteristics to a cheaper and perhaps stronger base material.

The present invention contemplates the formation and application of a lead-tin-antimony alloy layer by means of electrodeposition. Lead-tin alloys have been successfully.electrodeposited in the past but the electrodeposition of a ternary alloy such as a lead-tin-antimony alloy has encountered many difficulties. Attempts to electrodeposit ternary alloys have been hindered by suchdefects as the lack of control over the composition of the plate, poor corrosion of anodes, deposition of one of the metals upon the anode in the form of a sludge, the oxidation of one of the metals to form a precipitate in the bath and the crystallization of the alloy deposit on the cathode.

According to the present invention'lead-tinantimony alloys may be successfully electrodeposited by the use of a special electroplating bath and special techniques in the use of the bath. The electroplating of a bearing alloy comprising about 2 to 3% antimony, about 3% tin and the balance lead has been made possible 'byemploying the method of the present invention.

The electrode'position of lead-tin-antimony alloys is preferably conducted using a. binary anode containing about 3.5% tin and the balance lead, while the electroplating bath contains lead fiuoborate, tin fluoborate, boric acid, fluoboric acid and antimony trioxide or antimony 5 Claims. (01. 204-43) iiuoborate, making a bath of the following composition:

Tin0.'5 to 10 grams per liter and preferably 3 grams per liter.

Lead5'to 200 grams per liter and preferably '75 grams per liter.

Antimony-,About 0.1 .or more grams per liter and preferably saturated.

Free boric acid-1 to 40 grams per liter and preferably 25 grams per liter.

Sufficient fluoboric acid to make the pH less than 3 (colorimetric) and preferably less than 0.5.

Beta naphthol-about 1 gram per liter which is approximately equivalent to saturation.

The electroplating is run at a cathode current density of 5 to amperes per square foot and preferably 15 to 30 amperes per square foot while the anode current density is one fourth to one half that of the cathode current density. The temperature of the bath is preferably kept at 37 C. to 38 C. by thermostatic control.

Antimony is added to the plating solution in the form of a soluble compound such as antimony trioxide (Sb203) or as antimony fluoborate (Sb(BF4)3) rather than anodically. Although a small amount of antimony can be introduced into the bath anodically, the results are unsatisfactory because of difficulties encountered at the anode or anodes such as polarization, selective dissolution of the anode metals to cause sludge or slime formation and galvanic deposition of antimony.

The plating bath must be replenished from time to time with antimony either as the trioxide or the fluoborate, or both. This may be accomplished by filtering the electroplating bath through a filter loaded with theantimony salt. This is done intermittently when the antimony content of the bath runs low. Should a higher antimony content be desired in the deposit, then the plating solution should be circulated over the antimony salt quite frequently or continuously.

One of the main difliculties in lead-tin plating from a fiuoborate bath, especially where the de-- sired tin content of the deposit is low" (about 3%), lies in the close control of the tin content. The kind and amount of organic addition agent has a potent effect upon the ratio of lead and tin in the deposit. Due to the relatively small amounts of these addition agents required and the dimculty of controlling the amount present in the solution analytically, there was recognized a definite need for automatically controlling the amount of one of these addition agents in the solution.

According to the method of the present invention it has been found that an addition agent such as beta-naphthol, present in the solution to the full extent of its solubility, serves very well in keeping the tin depositing out on the cathode as fast as it is dissolved from the anode. Keeping the solution saturated at all times with the betanaphthol gives a more uniform rate of deposition of tin than when periodic additions of lesser amounts of beta-naphthol are made. In the conventional method of making periodic additions of beta-naphthol the tin content in the deposit decreases when the concentration of the betanaphthol drops, thus allowing tin to accumulate in the bath; then when an addition of beta naphthol is made, the thin content of the deposit becomes too high.

The beta-naphthol concentration may be maintained at saturation by continuously passing the electroplating bath through a filter loaded with bet-a-naphthol.

There is a tendency for galvanic deposition of antimony from the solution onto the lead-tin anode. This phenomenon is obviated by special wrappings around the anode, using paper next to the anode and gauze over the paper to hold the paper in place. This type of wrapping inhibits the free flow of the plating solution past the anode and minimizes the galvanic deposition of antimony from the solution onto the lead-tin anode. When current is flowing the anode dissolves electrolytically and keeps the paper wrapping saturated with lead and tin fiuoborates, thereby preventing antimony ions from coming in contact with the metal anode to a great extent. To increase the anode .area within a confined space such as in bearing plating where the anode must be positioned inside the bearing, it is preferred to employ the anode alloy in the form of shot held in a perforated non-metallic container having the wrapping around this container.

In the drawing, there is shown a tank H filled with electroplating bath 12 into which are im mersed the anode it? covered with wrapping 23 hanging from the positive bus bar hi and the article to be plated as the cathode l suspended from the negative bus bar E6. The electrolyte passes by way of conduit 18 through filter ill which is loaded with beta-naphthol and then is forced by pump 5 9 through conduit 20 back into the tank. The electrolyte also is drawn by way of conduit 2| by pump 22 which forces it through filter 23 which is filled with antimony trioXide or antimony fiuoborate. The electrolyte then passes into the tank by way of conduit 24.

Other organic addition agents besides betanaphthol such as glue, and aloin, either singly or in combination, may be added to the electroplating bath.

Tin often keeps oxidizing and precipitating out of the plating solution as metastannic acid. The oxidation of the tin may be stopped by placing a sheet of pure tin 2-5 in the bottom of the electroplating tank, or by use of tin shot in one of the filters.

Satisfactory lead-tin-antimony alloys may be electrodeposited in accordance with the present invention by close control of temperature, current density, circulation and bath composition. Good deposits of several thousandths or even several hundredths of an inch thick may be obtained with good control over the composition of the deposits. The plating bath and method of the present invention make possible rapid electroplating of smooth, dense deposits of substantial thickness which adhere tenaciously to metal bases of steel, copper, silver and other metals.

While specific embodiments of the invention have been described, it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. The method of electrodepositing lead-tinantimony alloys which comprise passing current between a cathode and an anode consisting of a minor portion of tin and a major portion of lead through an electrolyte consisting essentially of lead fiuorborate, tin fluoborate, and a compound selected from the group consisting of antimony trioxide and antimony fiuoborate, maintaining a tin concentration of 0.5 to 10 grams per liter and a lead concentration of 5 to 200 grams per liter by corrosion of the anode, and maintaining an antimony concentration of 0.1 gram per liter to saturation by periodic addition of a soluble compound to the bath, said compound being selected from the group consisting of antimony trioxide and antimony fluoborate.

2. A process in accordance with claim 1 in which the pH of the electrolyte is maintained below 3.0 by periodic addition of fluoboric acid.

3. A process in accordance with claim 1 in which the electrolyte is saturated with an addition agent selected from the group consisting of beta napthol, glue, and aloin.

4. A process in accordance with claim 1 in which the anode is provided with a wrapping of permeable fibrous material.

5. A process in accordance with claim 1 in which the anode is covered with a layer of paper and a layer of gauze is placed over the paper to hold the latter in place upon the anode.

JAMES M. BOOE.

REFERENCES CITED The following references are of record in the Australian; Feb. 21, 1945 OTHER REFERENCES Metal Industry, June 1920, pages 264466. Metal Industry, Apr. 1936, pages 469-471. 

